(define 3rd caddr)  ;only to understand 'acid' better

;-----------acid FINDS THE CORRESPONDING ACID FOR A rna-CODON
(define (acid ls) ;----it takes a list with 3 items
 (let ( (1st (car ls)) (2nd (cadr ls)) )
  (cond ((equal? 1st  'u)
           (cond ((equal? 2nd  'u)
                      (if (member? (3rd ls) 'uc) 'phe 'leu))
                 ((equal? 2nd  'c) 'ser)
		 ((equal? 2nd  'a)
		      (if (member? (3rd ls) 'uc) 'tyr 'stop))
                  ( (member? (3rd ls) 'uc) 'cys )
                   ( (equal? (3rd ls) 'a) 'stop )
                    (else 'trp) )) 
	((equal? 1st  'c)
           (cond ((equal? 2nd  'u) 'leu)
		 ((equal? 2nd  'c) 'pro)
		 ((equal? 2nd  'g) 'arg)
		  ( (member? (3rd ls) 'uc) 'his)
                   (else 'gln) ))
	((equal? 1st  'a)
	   (cond ((equal? 2nd  'u)
                      (if (equal? (3rd ls) 'g) 'met 'ile))
		 ((equal? 2nd  'c) 'thr)
	         ((equal? 2nd  'a)
		      (if (member? (3rd ls) 'uc) 'asn 'lys))
		   ( (member? (3rd ls) 'uc) 'ser)
                    (else 'arg) ))
         ( (equal? 2nd  'u) 'val)
         ( (equal? 2nd  'c) 'ala)
         ( (equal? 2nd  'g) 'gly)
          ( (member? (3rd ls) 'uc) 'asp)
           (else 'glu)  ))) 

       ;---STARTS THE PROCESS   
(define (locate_intron gen acids)
    (dna-to-rna gen acids))


           ;--------d-to-r FINDS THE CORRESPONDING RNA-NUCL. FOR A DNA-NUCL.
       (define (d-to-r n) ;;;#### "n" stands for "nucleotid"
	  (cond ((equal? n 'a) 'u)
		((equal? n 't) 'a)
                ((equal? n 'g) 'c)
		(else 'g)))
		    
     ;---CONVERTS THE WHOLE GEN LIST INTO THE rna-REPRESENTATION
(define (dna-to-rna gen acids)
     (control@ (map d-to-r gen) acids))

;========= HELPER DEFINITIONS ==============================================

          (define (first3 sent) ;---gives the first 3 items of a list
            (if  (<3? sent) '() ;---as a list
		(list (car sent) (cadr sent) (caddr sent)) ))

	  (define (last3 sent)  ;---gives the last 3 items of a list
            (if  (<3? sent) '() ;---as a list
                (last3! sent) ))

      (define (last1 lst)      ;---gives the last item of a list
        ( if (null? (cdr lst)) lst (last1 (cdr lst)) ))

      (define (last3! lst)  ;---assumes that the list is longer than 3 items 
        ( if (null? (cdddr lst)) lst (last3 (cdr lst)) ))

      (define (l-bl lst)    ;---the same as 'bl' only for lists
        ( if (null? (cdr lst)) '() 
               (cons (car lst) (l-bl (cdr lst))) ))
      (define (l-3bl lst)   ;---throws out the last 3 items of a list
        ( if (null? (cdddr lst)) '()
               (cons (car lst) (l-3bl (cdr lst))) ))

      (define (<3? lst)     ;---finds if a list contains less than 3 items
        ( or (null? lst) (null? (cdr lst)) (null? (cddr lst)) ))

      (define ( PATTERN WHATEVER THE LOCATION OF THEM IS
       ;--- {if there can exist of course}
(define (center! gen! acids! head! end! i!)
 ( if (= 2 i!) '()
   (let ( (soln ((if (zero? i!) exon1 exon3) gen! acids! head! end! i!)) )
    ( cond 
        ( (null? soln)
              (center! gen! acids! head! end! (+ i! 1)) )
        ( else soln ) )) ))



       ;---FINDS WHERE INTRON1 STARTS  
(define (exon1 gen acids head end i)
  (cond 
      ( (<3? gen)  '())
      ( (zero? i)             ;************* BEFORE EXON3   i=0
          (cond( (null? (cddr acids)) 
                   (exon3 gen acids head end i) )
               ( (equal? (acid (first3 gen)) (car acids))
                   (exon1 (cdddr gen) (cdr acids) (+ head 3) end i) )
	       ( else (exon3 gen acids head end i) )) ) 
      ( (null? (cdr acids))   ;************* AFTER  EXON3   i=1
          (exon2 gen acids head end) )
      ( (equal? (acid (first3 gen)) (car acids))
          (exon1 (cdddr gen) (cdr acids) (+ head 3) end i) )
      ( else (exon2 gen acids head end) ) )) 


       ;---FINDS WHERE INTRON2 ENDS
(define (exon3 gen acids head end i)
  (cond 
      ( (<3? gen)  '())
       ( (zero? i)            ;************* AFTER  EXON1   i=0
          (cond( (null? (cdr acids))      
                   (exon2 gen acids head end) )
               ( (equal? (acid (last3 gen)) (last acids))
                   (exon3 (l-3bl gen) (l-bl acids) head (- end 3) i) )
               ( else (exon2 gen acids head end) )) ) 
      ( (null? (cddr acids))  ;************* BEFORE EXON1   i=1
          (exon1 gen acids head end i) )
      ( (equal? (acid (last3 gen)) (last acids))
          (exon3 (l-3bl gen) (l-bl acids) head (- end 3) i) )
      ( else (exon1 gen acids head end i) ) )) 



       ;---INVOKES scrutinize WITH NECESSARY ARGUMENTS
(define (exon2 gen acids head end)
  (scrutinize  gen acids (* 3 (length acids)) head head end) )

       ;---SEARCHES EXON2 OVER THE REMAINING GEN
(define (scrutinize gen acids exon2l head1 end1+ end2)
  (cond                             ;;### exon2l stands for "EXON2 Length"
        ( (